Thin and ultra-thin semiconductor substrates, such as semiconductor wafers or foils with a thicknesses in the range of a fraction of micron up to 100 microns, are highly advantageous in many applications including but not limited to high-performance semiconductor microelectronics, system-on-a-chip (SOC), silicon-on-insulator (SOI), MEMS, power electronics, flexible ICs, solar photovoltaics, and optoelectronics.
Further, crystalline (both mono-crystalline and multi-crystalline) silicon (c-Si) wafers are widely used in producing silicon based photovoltaic solar cells, mainly due to higher efficiencies and synergies with the well-established silicon microelectronics industry infrastructure and supply chain. The trend in the mainstream c-Si wafer solar cell industry has been to scale down wafer thicknesses to below 200 microns in order to reduce the amount of silicon material in grams used per watt of solar cell rated peak power—thus reducing the overall manufacturing cost of the solar photovoltaic power modules. For example, the leading edge monocrystalline silicon wafer solar cells are projected to scale down to a wafer thickness of about 120 microns by 2012, from a current wafer thickness of 140 to 200 microns. Technologies are also being developed that use less than 100 microns (μm) c-Si foil (such as foils in the thickness range of a few microns to below 50 microns) to make cost-reduced high efficiency solar cells. In addition, thin semiconductor substrates or foils may be a requirement to make partially see-through c-Si solar cells for building integrated photovoltaic (BIPV) products.
However, thin c-Si solar cells are usually much larger than other stand-alone thin semiconductor or MEMS devices (chips): over 200 to 500 cm2 for solar cells vs. less than 1 to several cm2 for semiconductor microelectronic and MEMS chips. Typical silicon solar cell sizes are 210 mm×210 mm, 156 mm×156 mm, and 125 mm×125 mm squares (or pseudo squares).
Semiconductor wafers, such as monocrystalline silicon wafers are quite brittle and break easily from stresses, micro-cracks, and edge damage when their thickness is reduced—particularly to much less than 150 microns. In addition, because of the reduced mechanical rigidity of a thin wafer it becomes more flexible and behaves more like a flexible piece of thin foil. As a result, it is rather difficult and problematic (in terms of mechanical yield) to handle and process these thin wafers in normal automated semiconductor microelectronic or photovoltaic process equipment and fabs that are designed to process and handle wafers with regular thicknesses (e.g., ˜150 microns to ˜1000 microns).
In order to use existing commercially-available wafer processing equipment and fab automation solutions for thin wafer handling and processing, mobile chucks or carriers have been developed to support and hold thin wafers and substrates in place during handling and processing. Using these carriers, the bonding of the thin wafer and the carrier may be made either temporary or permanent. Many current thin wafer bonding techniques are too expensive and cumbersome (e.g., bonding and de-bonding steps take too long and use expensive materials and/or processes) to be used for mass production of low-cost solar cells.
Current mobile electrostatic carriers (MESC) have been developed to utilizing electrostatic force between two electrodes to hold the thin wafers. Generally, there are two types of MESCs: a unipolar (monopolar) type and a bipolar type. FIGS. 1A (prior art) and 1B (prior art) are cross-sectional schematic drawings of current designs of a unipolar MESC and a bipolar MESC, respectively. Unipolar MESCs consists of an electrode layer embedded in a dielectric material, shown the electrode extends along the entire lateral plane of the MESC. In this configuration the thin wafer to be clamped forms the second electrode of the capacitor, which means the thin wafer surface has to be electrically contacted for charging/clamping and discharging/declamping. As shown in FIG. 1A, unipolar MESC 10 comprises metal (or electrically conductive material) base-plate 12 under thin dielectric layer 14. The metal (or electrically conductive material) base-plate is maintained at a high-voltage relative to thin wafer 16 sitting on top of the thin dielectric layer to create an electrostatic force which clamps the thin wafer to it. In other words, the thin wafer serves as one of the two capacitor electrodes—the other being the base-plate—when a high voltage is applied to activate the chucking and when the MESC is discharged.
Unipolar MESCs are often made from the same material as the thin wafer to minimize or eliminate the coefficient of thermal expansion (CTE) mismatch during thermal process. The advantage of such a unipolar MESC is its simplicity, however when a dielectric layer or a thick non-conductive reinforcement layer is applied onto the thin wafer front surface, it is difficult to discharge the capacitor in order to separate the thin wafer from the MESC since there is no conductive path access to the thin wafer (particularly if the thin wafer goes through a dielectric deposition process such as deposition of a PECVD silicon nitride passivation/ARC layer in a silicon solar cell).
Current bipolar MESCs consist of two electrodes embedded and laterally insulated in a dielectric material. In contrast to a unipolar MESC, the thin wafer does not need to be electrically contacted for charging and discharging because the capacitor is formed between the two electrodes or multiple pairs of electrodes. Such a bipolar MESC is usually made from metal electrodes and polymer dielectric layers; therefore it is limited in terms of thin wafer thermal process and wet chemical process capabilities. As shown in FIG. 1B, bipolar MESC 20 has both of electrodes of opposite polarity (negative electrodes 22 and positive electrodes 28) embedded under dielectric layer 24 and in the MESC itself. This bipolar MESC design relies upon the electric field generated between the two electrodes to hold thin wafer 26 in place. When using a bipolar MESC, during the chucking and dechucking, the thin wafer does not need to be electrically contacted.
Current bipolar mobile electrostatic carriers are often made from metallic electrodes and polymer dielectric layers, because of which the overall performance of the MESCs is limited with some of the following concerns:                (1) The existence of metal and polymer limits the thin wafer processing temperature to be typically less than 300° C., which means that current MESCs cannot be reliably used for wafer processing much above 300° C.;        (2) The thin wafer and processing equipment may be contaminated by the MESC structural materials, especially when processed at elevated temperatures;        (3) The thermal (TCE) mismatch between the MESC structural materials (metal & polymer) and the thin semiconductor wafer may cause warpage or even breakage of the thin wafer (and/or formation of microcracks);        (4) The MESC structural materials (metal & polymer) may not be chemically compatible with commonly used dry and wet chemical etching and deposition processes;        (5) The overall mobile electrostatic carrier lifetime may be affected by the dielectric qualities of the polymer dielectric materials, especially in wet environments; and        (6) De-clamping of thin wafer from the MESC may be difficult and take a long time (particularly after high-temperature processing) due to the charging of the MESC capacitor dielectric.        